Plasma gasification reactors (sometimes referred to as PGRs) are a type of pyrolytic reactor known and used for treatment of any of a wide range of materials including, for example, scrap metal, hazardous waste, other municipal or industrial waste and landfill material, and vegetative waste or biomass to derive useful material, e.g., metals, or a synthesis gas (“syngas”), or to vitrify undesirable waste for easier disposition. In the present description “plasma gasification reactor” and “PGR” are intended to refer to reactors of the same general type whether applied for gasification or vitrification, or both. Unless the context indicates otherwise, terms such as “gasifier” or “gasification” used herein can be understood to apply alternatively or additionally to “vitrifier” or “vitrification”, and vice versa.
PGRs and their various uses are described, for example, in Industrial Plasma Torch Systems, Westinghouse Plasma Corporation, Descriptive Bulletin 27-501, published in or by 2005; a paper by Dighe in Proceedings of NAWTEC16, May 19-21, 2008, (Extended Abstract #NAWTEC16-1938) entitled “Plasma Gasification: A Proven Technology”; a paper of Willerton, Proceedings of the 27th Annual International Conference on Thermal Treatment Technologies, May 12-16, 2008, sponsored by Air & Waste Management Association entitled “Plasma Gasification—Proven and Environmentally Responsible” (2008); U.S. Pat. No. 7,632,394 of Dighe et al., issued Dec. 15, 2009, entitled “System and Process for Upgrading Heavy Hydrocarbons”; a U.S. Patent Application of Dighe et al., Ser. No. 12/157,751, filed Jun. 14, 2008, entitled “System and Process for Reduction of Greenhouse Gas and Conversion of Biomass”, (Patent Application Publication No. 2009/0307974, Dec. 17, 2009), and Dighe et al. patent application Ser. No. 12/378,166, filed Feb. 11, 2009, entitled “Plasma Gasification Reactor”, (Patent Application Publication No. 2010/0199557, Aug. 12, 2010), all of said documents being incorporated by reference herein for their descriptions of PGRs and methods practiced with them.
It is known to set up and operate such PGRs with a carbonaceous bed in a lower part of a reactor vessel where the bed is arranged with plasma torches that elevate the bed temperature (e.g., to at least about 1000° C.) for thermal reaction with added material that is to be gasified or vitrified. Although there have been suggestions that carbon material for such a carbonaceous bed can be of a variety of carbon bearing materials, there has in the past been a heavy reliance on the use of coke for such purposes as it is about 90% pure carbon and has chemical, thermal, and strength properties that are favorable for many processes that are performed in such reactors. “Coke” is a term for a product of a fossil fuel e.g., coal or petroleum, subjected to drying, e.g., by baking, to drive off volatile constituents.
While the carbonaceous bed is an important component in the operation of a PGR, another known form of a gasification reactor is a gasifier utilizing a carbonaceous bed (of coke) but without utilizing plasma torches. The carbonaceous bed of such a reactor serves all the same functions as it does in a PGR with respect to the distribution of gases and the movement of molten materials. However, in addition, the carbonaceous bed also serves to provide the thermal energy for gasification that would otherwise be provided by a plasma torch. A carbonaceous bed of such a reactor may be initially activated to a temperature for gasification by, for example, brief ignition of natural gas supplied to the bed.
Among the desirable criteria of the carbonaceous bed of PGRs and other reactors is that it be of particles irregular enough in shape to leave voids allowing gases to flow to the surface of the particles where reactions occur and gaseous reaction products to rise from the bed. The voids also allow molten metals and other liquids resulting from the process performed in the reactor to flow down to a metal and slag exit port. Voids of the bed and the porosity of particles of the bed can contribute to desirable reactions and flow characteristics. Coke allows the formation of such a bed and has sufficient strength of the particles for many processes not to be crushed during operation by the burden of working material deposited on top of the bed.
Despite the satisfactory performance that coke very often provides, it is sometimes the case that factors such as the expense of coke and concerns about its manufacture and use impacting the environment, as it is a fossil fuel, may prevent or limit its use in some processes at some reactor sites.
In the known prior art, U.S. Pat. No. 4,828,607 issued May 9, 1989, to Dighe et al., and entitled “Replacement of Coke in Plasma-Fired Cupola”, discloses a process that includes providing coal instead of coke, although still a fossil fuel, along with metal scrap and a fluxing material, to a plasma-fired cupola to produce iron or ferro-alloys. This evidences fairly early interest in minimizing coke usage in such applications although coke still remains the only form of carbon material that is widely used in operating reactors with carbonaceous beds. Wood or wood products (e.g., charcoal) are known carbon sources but have not found practical application as significant coke replacements in pyrolytic reactors.